1. Field of the Invention
The invention to relates to liquid scorch inhibitor compositions for polyurethane foam compositions.
2. Discussion of the Prior Art
Those familiar with the manufacture of flexible polyurethane slabstock-type foams have observed the phenomenon known as scorch, which is a manifestation of polymer degradation. Visually, scorch appears as a darkening in the interior of a polyurethane foam bun that can be quantitatively characterized in hue, value and chroma. Scorch is a well understood thermo-oxidative degradation process which is generally the result of free-radical reactions involving the polyurethane matrix, polyetherpolyol, isocyanate (MDI or TDI) and additives within the foam. For example, at elevated temperatures (200° C., 392° F.), 4,4′-diphenylmethane diisocyanate-based thermoplastic polyurethane elastomers suffer main chain rupture. Heat is also known to cause other homolytic degradation processes, such as hydrogen atom abstraction from carbon alpha to the ether linkage. Generated free radicals combining with oxygen lead to hydro-peroxide species, which further cleave and propagate degradation. Expectedly, polyols based upon polypropylene oxide glycols are less stable than those based upon polyethylene glycol due to stability differences of alpha carbon free radical. Thus, scorch degradation follows well understood reaction pathways. Historically CFCs have been used in foam formulations to remove excessive heat build up and thereby diminish the scorch effect. However, because of reductions in chlorofluorocarbons (CFCs) and, more currently, transitional hydrochlorofluorocarbons (HCFC), coupled with increases in water to compensate (up to 6 pph) in foam formulations, scorch continues to be of great concern to the industry.
To counter scorch or destructive free radical reactions, antioxidants are added. A review of the patent literature shows diversity of antioxidant formulation strategies.
U.S. Defensive Publication No. T968,002 to Baxter teaches blends of aromatic amines, hydroquinone (structure 1 below) and phosphite to produce a functioning scorch inhibitor. U.S. Pat. No. 4,010,211 to Preston discloses the use of an effective amount of a select diphenylamine derivative, alone or in further combination with hydroquinone, which serves to inhibit scorching in the polyurethane foam. U.S. Pat. No. 6,676,849 to DeMassa teaches a blend of tert-amyl-hydroquinone with commercial phenolics. Thus, hydroquinone and derivatives have been used in various scorch inhibitor blends.
Current commercially available conventional compositions follow the teachings of U.S. Pat. No. 4,275,173 to Hinze and U.S. Pat. No. 4,444,676 to Statton, but other blends have also appeared varying upon this theme. U.S. Pat. No. 4,933,374 to Suhoza reports a similar effective scorch inhibitor blend consisting of (a) tetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)]methane and (b) a reaction product of diphenylamine and diisobutylene, wherein the molar ratio of the amine to the diisobutylene is 1:1.1 to 1:2.5. The stabilizer composition may contain a third synergistic component, phenothiazine, or certain alkyl derivatives of phenothiazine.
Structure 1. Structure II 
We have now discovered that a liquid blend comprised as follows is surprisingly superior as a scorch inhibitor composition for polyurethane foams to the above discussed blends:
(a) one or more derivatized phenolic compounds
(b) one or more aromatic amines in the form of a liquid
(c) a substituted hydroquinone, and
(d) optionally, phenothiazine.